Opengl texture cylinder trouble - c++

Question

I'm a little bit confused.
I want to texture a "cylinder", so my first approach was this (with n the number of faces and k the number of iteration)
void cylindrer(double r, int n,double h){
double x[n],y[n];
for(int k=0; k<n; k++){
x[k]=r*cos(2*k*M_PI/n);
y[k]=r*sin(2*k*M_PI/n);
}
glEnable(GL_TEXTURE_2D);
glBindTexture(GL_TEXTURE_2D, texObject[1]);
for(int k=0; k<n ;k++){
int m= (k+1)%n;
glBegin(GL_POLYGON);
glTexCoord2f(1.0/n*(k),0.0); glVertex3f( x[k], y[k], h/2);
glTexCoord2f(1.0/n*(k),1); glVertex3f( x[k], y[k], -h/2);
glTexCoord2f(1.0/n*(k+1),1); glVertex3f( x[m], y[m], -h/2);
glTexCoord2f(1.0/n*(k+1),0.0); glVertex3f( x[m], y[m], h/2);
glEnd();
}
glDisable(GL_TEXTURE_2D);
}
The texture is applied but reverse so I'd changed to
glBegin(GL_POLYGON);
glTexCoord2f(1.0/n*(n-k), 0.0); glVertex3f( x[k], y[k], h/2);
glTexCoord2f(1.0/n*(n-k), 1.0); glVertex3f( x[k], y[k], -h/2);
glTexCoord2f(1.0/n*(n-k-1), 1.0); glVertex3f( x[m], y[m], -h/2);
glTexCoord2f(1.0/n*(n-k-1), 0.0); glVertex3f( x[m], y[m], h/2);
glEnd();
And it works and look like this :
when I use this texture :
But now I want to rotate the texture of 90 degrees, so I create a new jpeg file and rotate it.
So the texture now look like this :
But this is the result when I use it :
The texture is twisted around the cylinder and I don't understand why.
Any idea ?
How I load texture :
#include <math.h>
#include <jpeglib.h>
#include <jerror.h>
GLuint texObject[2]; // textures
int main(int argc,char **argv){
glutInit(&argc,argv);
glutInitDisplayMode(GLUT_RGB | GLUT_DOUBLE | GLUT_DEPTH);
glutInitWindowPosition(200,200);
glutInitWindowSize(1366,768);
glutCreateWindow("");
glClearColor(0.0,0.0,0.0,0.0);
//glColor3f(1.0,1.0,1.0);
glShadeModel(GL_FLAT);
glPointSize(2.0);
glEnable(GL_DEPTH_TEST);
glGenTextures(2, texObject);
loadJpegImage("./textureCorps5.jpg", 1);
glutMainLoop();
return 0;
}
void loadJpegImage(char *fichier, int i)
{
struct jpeg_decompress_struct cinfo;
struct jpeg_error_mgr jerr;
FILE *file;
unsigned char *ligne;
cinfo.err = jpeg_std_error(&jerr);
jpeg_create_decompress(&cinfo);
#ifdef __WIN32
if (fopen_s(&file,fichier,"rb") != 0)
{
fprintf(stderr,"Error\n");
exit(1);
}
#elif __GNUC__
if ((file = fopen(fichier,"rb")) == 0)
{
fprintf(stderr,"Error\n");
exit(1);
}
#endif
jpeg_stdio_src(&cinfo, file);
jpeg_read_header(&cinfo, TRUE);
unsigned char image[cinfo.image_width*cinfo.image_height*3];
jpeg_start_decompress(&cinfo);
ligne=image;
while (cinfo.output_scanline<cinfo.output_height)
{
ligne=image+3*cinfo.image_width*cinfo.output_scanline;
jpeg_read_scanlines(&cinfo,&ligne,1);
}
jpeg_finish_decompress(&cinfo);
jpeg_destroy_decompress(&cinfo);
glBindTexture(GL_TEXTURE_2D, texObject[i]);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB, cinfo.image_width, cinfo.image_height, 0,
GL_RGB, GL_UNSIGNED_BYTE, image);
glTexEnvf(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_DECAL);
}

Answer 1

GL_UNPACK_ALIGNMENTspecifies the alignment requirements for the start of each pixel row in memory. By default GL_UNPACK_ALIGNMENT is set to 4.
This means each row of the texture is supposed to have a lenght of 4*N bytes.
Your texture is an RGB texture, which needs 24 bits or 3 bytes for each texel and you tightly packed the texels and especially the lines of the texture. This means that you may disregard the alignment of 4 for the start of a line of the texture (Except 3 times the width of the texture is divisible by 4 without a remaining).
To deal with that you have to change the alignment to 1. This means you have to set glPixelStorei(GL_UNPACK_ALIGNMENT, 1); before glTexImage2D, for reading a tightly packed texture.
Otherwise you will get an offset per line of 0-3 bytes when reading the texture. This causes a continuously twisted texture.
Instead you can take care of the alignment and the aligned length of a line when you create the turned texture, too:
int bytesPerLine = cinfo.image_width * 3;
bytesPerLine += bytesPerLine % 4;
unsigned char image[bytesPerLine * cinfo.image_height];
jpeg_start_decompress(&cinfo);
while (cinfo.output_scanline<cinfo.output_height)
{
ligne = image + bytesPerLine*cinfo.output_scanline;
jpeg_read_scanlines(&cinfo,&ligne,1);
}
jpeg_finish_decompress(&cinfo);
jpeg_destroy_decompress(&cinfo);